The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Capture of CO2 emissions from coal-based energy systems is one means in which the greenhouse gas intensity of coal utilisation can be reduced. Capture can be performed post-combustion or pre-combustion, in which the energy content of coal-derived syngas is shifted via the water-gas-shift (WGS) reaction to H2, followed by the separation and capture of CO2 as shown in (1):CO+H2O⇄CO2+H2  (1)
Membrane-based separation technology is currently under development for the separation of H2 from mixed gas streams. Broadly speaking, a membrane is a near two-dimensional structure which is selectively permeable to one species. In the context of gas separation, a membrane allows one species to selectively permeate (usually H2), while blocking other species (e.g. CO, CO2, H2O, N2 etc.). Hydrogen-selective membranes can be created from inorganic, metallic or ceramic materials, each of which has characteristic hydrogen throughputs, operating temperatures and selectivity.
A catalytic membrane reactor (CMR) integrates a hydrogen selective membrane with a water gas shift catalyst, thereby enabling the production and separation of H2. CMRs typically operates between at about 450° C. for favourable fast kinetics for the WGS reaction. Furthermore, the CMR allows greater-than-equilibrium conversion to be attained, as the continuous depletion of the H2 product through the membrane pushes the WGS reaction to the product side. The continuous extraction of H2 in situ can allow CO conversions of almost 100%.
Palladium is the best known alloy membrane material, having an ability to permeate hydrogen between 300 to 600° C. whilst being tolerant to syngas species such as CO and H2O. However, the high cost of palladium (˜$US 330/m2/μm (2014)), has driven research towards minimising its consumption, most notably through alloying with less-expensive metals, and minimising thickness by depositing very thin (<5 μm) layers on support structures with very fine pores.
A number of other metals exhibit very high hydrogen permeability, most notably vanadium, titanium, tantalum and zirconium. At 400° C., the hydrogen permeability of these metals is around two orders of magnitude greater than palladium, and the raw materials prices are significantly lower.
Of these metals, V has the widest alloying range, which means it has the widest scope for modifying the alloy properties to meet the demands of a CMR. However, vanadium exhibits poor mechanical stability under hydrogen making it unsuitable for use in industrial hydrogen separation processes. Vanadium tends to absorb hydrogen in high concentrations. As a consequence, the vanadium hydride is prone to brittle failure because hydrogen negatively influences the metallic bonding within the alloy.
Any material used for a H2-selective membrane in a CMR must also have suitable formability/mechanical properties which allow the material to be fabricated into a desired configuration, such as planar or tubular membranes. For example, alloys used for the manufacture of tubular products normally exhibit a maximum elongation of 25 to 35% in mass production (via extrusion and drawing), and at least 10 to 20% if using a customised deformation process (with reduced deformation per pass and extensive anneals between passes).
U.S. Pat. No. 7,001,446 B2 discloses hydrogen-permeable membranes for separation of hydrogen from hydrogen-containing gases. The membranes are multi-layer having a central hydrogen-permeable layer with one or more catalyst layers, barrier layers, and/or protective layers. The central hydrogen-permeable layer is taught in some embodiments as comprising a hydrogen permeable metal or metal alloy, including alloys of vanadium. A large number of suitable alloys are generally taught being suitable for this hydrogen-permeable layer, for example as outlined in Table 1 of this US patent. However, it is noted that no specific alloy is taught as providing optimal properties for use as a membrane, and more particularly a tubular membrane suitable for a CMR.
It would therefore be desirable to provide an alloy and a membrane formed using said alloy having appropriate hydrogen transport, mechanical stability and formability for use in a CMR, preferably as a tubular membrane for a CMR.